Integrated circuits are manufactured as assemblies of various devices, such as transistors that make up a chip. In the process of manufacturing integrated circuits, after the individual devices, such as transistors, have been fabricated in the silicon substrate, the devices must be connected together to perform the desired circuit functions. This connection process is generally referred to as “metallization”, and is performed using a number of different photolithographic and deposition techniques.
Contact plugs are employed to make a solid connection between an underlying device, for example, and an overlying interconnection conductive line, for example. The fabrication of a contact typically involves forming an opening in the dielectric layer and filling the opening with a metallic layer, such as aluminum or tungsten. However, aluminum or tungsten ions from the contact can migrate into a silicon substrate through a doped region, causing a short to the substrate. To minimize this shorting, many processing techniques deposit a barrier layer before depositing the aluminum or tungsten. One type of common barrier material is titanium nitride (TiN). While titanium nitride has a good barrier ability, it needs to be thick enough to effectively function as a barrier layer. As integrated circuit devices are defined more finely, the diameter of the contact shrinks and becomes more critical. Thus, a thick titanium nitride barrier metal layer is less desirable in more highly integrated circuits.
It has been found that increasing the nitride in titanium nitride improves its barrier effect. One approach is to implant nitrogen into titanium nitride in order to increase the barrier effect and reduce the thickness of the titanium nitride barrier metal layer in order to meet the highly integrated structure requirement. One method for implanting the nitrogen into titanium nitride is to perform a nitrogen plasma treatment in an atmosphere of hydrogen gas.
Another commonly used barrier layer is formed from metal organic CVD titanium nitride (MOCVD-TiN). Inherently, the MOCVD-TiN material contains impurities such as carbon and oxides, so that the resistance of MOCVD-TiN material is high. In order to reduce the resistance, one method removes these impurities by treating the barrier layer with a plasma gas containing an atmosphere of nitrogen. However, following the plasma gas treatment, the thickness of the MOCVD-TiN is substantially reduced. Consequently, the treated MOCVD-TiN layer thus formed has comparatively lower resistance, but the thickness is not adequate to function as a barrier layer effectively.
The effectiveness of the contact is limited by the contact resistance between the barrier metal layer and the doped regions in the substrate. Contact resistance is of particular concern in CMOS (complementary metal-oxide-silicon) technology. One approach to reduce the contact resistance is to deposit a conformal refractory metal layer into the opening, deposit the barrier metal on the refractory metal layer, and then perform a thermal process to effect a reaction between the metal and the silicon atoms of the substrate to form metal silicide. Since the metal silicide has a low resistance, consequently the contact resistance can be reduced.
One of the concerns of this methodology is that the plasma treatment to condense or reduce contaminants in the MOCVD-TiN layer affects the contact metal (titanium) underneath, especially in the case when titanium silicide formation is required for low contact resistance to the silicon substrate. This is due to portions of the titanium being easily converted to titanium nitride prior to being converted to titanium silicide in a subsequent rapid thermal anneal (RTA) step. The conversion of portions of the titanium to titanium nitride causes degradation of the contact resistance and distribution. Furthermore, in order to provide adequate titanium to form silicide, and compensate for the conversion of a portion of the titanium to titanium nitride, an excessive amount of titanium needs to be initially deposited. However, this raises concerns regarding overhang on the contact opening, as well as requiring additional titanium material for the process.